In the fabrication of semiconductor devices, numerous conductive regions are formed in a semiconductor substrate, and various conductive layers are formed on a semiconductor substrate. The conductive regions and layers are isolated from one another by a dielectric, for example, a silicon oxide layer (preferably silicon dioxide). The silicon oxide may be grown or deposited by physical deposition or by a variety of chemical deposition methods. Additionally, the silicon oxide may be undoped or doped, for example, with phosphorous to form phosphosilicate glass (PSG). The method of forming the silicon oxide layer and the doping of the silicon oxide layer will depend on various device and processing considerations.
During the manufacture of semiconductor devices, photoresists, which are photosensitive films used for the transfer of images to a multilayer structure, are formed on the multilayer structure, such as a silicon oxide layer of a multilayer structure. The photoresist layer is then exposed through a photomask to a source of activating radiation. The photomask has areas which are opaque to activating radiation and other areas which are transparent to activating radiation. Exposure to activating radiation provides a photoinduced chemical transformation of the photoresist coating to thereby transfer the pattern of the photomask to the photoresist coated substrate. Following exposure, the photoresist is developed to provide a relief image which permits selective processing of the underlying structure.
At several stages during fabrication, it is necessary to make openings in the dielectric to allow for contact to underlying regions or layers. Generally, an opening through a dielectric layer exposing a diffusion region of the substrate or an opening through a dielectric layer between polysilicon and the first conductive layer (i.e., the conductive layer closest to the substrate) is called a "contact opening." An opening in a silicon oxide layer formed elsewhere is generally referred to as a "via." As used herein, an "opening" will be understood to refer to any type of opening through any type of silicon oxide layer, regardless of the stage of processing, the layer exposed, or the function of the opening.
To form openings, a patterning layer of photoresist, which has openings corresponding to the regions of the silicon oxide layer openings to be formed, is formed over the silicon oxide layer. In most modern processes, a dry etch is performed wherein the wafer is exposed to a plasma, which is formed in a flow of one or more gasses called the etchant gas. One or more compounds are used as the etchant gas. For example, CF.sub.4, CHF.sub.3, SF.sub.6, and other gases may be used as, or as part of, the etchant gas. In addition, gases such as O.sub.2, Ar, N.sub.2, and others may be added. The particular gas mixture used will depend on the characteristics of the silicon oxide being etched and the desired etch characteristics such as etch rate, wall slope, and anisotropy.
In addition to the composition of the etchant gas, other factors influence the etch characteristics. These other factors include temperature, pressure, and gas flow rate, among others. These factors, as well as the composition of the etchant gas, may be varied to achieve the desired etch characteristics. There are invariably trade-offs between the various characteristics and the quality of the resulting etched structure. For instance, it is sometimes desirable to etch a multilayer structure with a high selectivity to silicon oxide as opposed to silicon nitride. Also, it may be desirable to etch a relatively thick layer of silicon oxide (especially doped silicon oxide). The process conditions that are designed to perform these two functions usually employ an etchant with a high carbon-to-fluorine ratio in a high-density plasma. More specifically, an etchant having a high carbon-to-fluorine ratio selectively etches silicon oxide over nitrogen-containing layers. Also, such an etchant, together with high-density plasma conditions, results in an aggressive etch rate capable of etching a relatively thick layer of silicon oxide.
One of the trade-offs mentioned above results from the fact that conditions selective to etching silicon oxide create an increased amount of polymer by-product deposition on the surface of the photoresist. The deposited polymer by-product is beneficial because it minimizes etching of the photoresist itself (which is undesirable). On the other hand, the presence of the by-product deposited on the photoresist contributes to the formation of detrimental "blisters" that occur on the surface of the photoresist. Blistering occurs when volatile constituents evolve from layers below the polymer by-product at a rate faster than the volatile constituents can diffuse through the polymer by-product.
The use of high-density plasma conditions accelerates etch rate and increases wafer temperature. These conditions can accelerate, in turn, the evolution of volatile constituents both from the photoresist and from the underlying layers (e.g., the silicon oxide layer itself). The combination of by-product deposition and the increased-rate volatilization combine to create kinetic problems; in particular, the rate of volatile evolution within the resist and the underlying layers is greater than the rate that the volatiles can diffuse through the deposited by-product layer out to the plasma atmosphere. In addition, relatively wide areas of unopened or unpatterned photoresist between the openings in the resist also contribute to blistering because the gas escape route to the side walls is longer in this event (i.e., a higher percentage of volatiles would not reach the side walls). The result is blistering or lift off of the deposited polymer layer, or both. Such blistering interferes with the ability of the etch process to generate the desired structure through the silicon oxide layer and with the ability to further process the etched wafer to generate a reliable product.
Accordingly, there remains a need for etching processes which avoid the problems associated with volatiles evolving during etching, particularly during etching conditions which result in the formation of increased polymer by-product. Such etching conditions typically include high-density plasma etching and the use of an etchant gas having a high carbon-to-fluorine ratio.